The present invention relates to circuits, and in particular, to a variable feedback circuit.
FIG. 1 is an example of a prior art amplifier circuit 100. Circuit 100 in this example includes an amplifier 101 having first and second input terminals (e.g., positive and negative inputs) and an output terminal. In this example, amplifier 101 includes a first resistor R1 coupled between an input signal Vin and the first negative input terminal. A second resistor R2 is coupled between the output terminal of the amplifier and the first input terminal. The second positive input terminal is coupled to ground. This amplifier configuration is the commonly-known inverting amplifier. The gain of this stage is given by the following equation:Vout/Vin=−R2/R1.In this example, amplifier 101 is powered by a +5 volt positive supply voltage and a zero volt (i.e., ground) negative supply voltage. Thus, if R2 is ten times the value of R1, then the circuit will have a gain of ten (10). Therefore, if the input signal has a peak-to-peak amplitude of 600 mV, then the output, Vout, should be a signal having a peak-to-peak amplitude of 6V. This example highlights a limitation of non-ideal circuits. In particular, since the positive supply is only 5V, the voltage on the output terminal, Vout, cannot achieve a voltage greater than 5V (typically less for amplifiers that do not have rail-to-rail output swing capability). When the input is sufficiently large in amplitude to cause the voltage on the output to reach the upper and/or lower power supply voltages (i.e., the upper and lower rails), the voltage on the output terminal will increase to the rail and then stop. This is referred to as “clipping.” A clipped output signal, Vout, is plotted to the right of the amplifier circuit in FIG. 1. As shown in the plot, the top and bottom of the signal are flat, indicating that the input signal and gain have cause the voltage on the output to reach a maximum value as constrained by the upper and lower rails.
Clipping is problematic in many circuit applications because during the period of time that the signal is clipped, the information contained in the input signal is lost. Clipping is particularly problematic in applications that are sensitive to distortion. For example, in audio applications it is desirable to minimize the amount of amplifier distortion because even the slightest distortion may be discemable to the human ear.
Thus, there is a need to reduce distortion in electronic circuits. The present invention solves these and other problems by providing variable feedback circuits and methods.